The invention relates to a gas-insulated line in a grid for transporting electrical power, the line comprising a conductor placed inside a case filled with a dielectric gas under pressure.
In such a line, electricity is transported by the conductor while the case is placed at zero potential. The nature and the pressure of the dielectric gas are selected as a function of the voltage which exists between the conductor and the case so as to guarantee electrical insulation. By way of example, a case having a diameter of about 1 meter (m) and filled with a mixture of nitrogen (N.sub.2) containing 2 percent (%) to 5% sulfur hexafluoride (SF.sub.6) at a pressure of 12 bars enables an aluminum conductor transporting power of the order of 1000 megawatts (MW) to 2000 MW at a voltage of about 400,000 volts (V) to be insulated. It will thus be understood that the zero potential of the case and its small size mean that the gas-insulated line is used on the ground or even buried. In this way, developing or renovating electrical power transport grids by means of gas-insulated lines makes it possible to provide better environmental protection.
It is well known that loads such as evacuated transformers fed by a distribution network itself connected to the transport grid give rise to reactive power. In power transport grids using overhead lines, a power capacitor is electrically connected to the line conductor to compensate the reactive power consumed by the loads, thus making it possible to reduce the amount of reactive power transiting via the grid to the benefit of active power transit. Power capacitors enable the transport capacity of the grid to be used more effectively and they limit the appearance of electrical voltage instabilities when the grid is being used to the limit of its capacity.
Overhead electricity lines have little inherent capacitive behavior so their contribution to supplying the transport grid with capacitive reactive power remains limited compared with the contribution of power capacitors.
Unlike overhead lines, gas-insulated lines have the advantage of directly providing capacitive reactive power to the power transport grid. Their capacitive behavior is about four times greater than that of an overhead line. By way of example, a 2000 MW overhead line transporting electricity at 400,000 V presents linear capacitance of about 13,000 picofarads per kilometer (pF/km), whereas for a gas-insulated line, under the same electrical voltage and power conditions, the linear capacitance is 50,000 pF/km. However, in the event that the reactive power supplied by the gas-insulated line is not sufficient on its own to compensate for the inductive reactive power load on the transport grid, as is frequently the case, it is still necessary to connect a power capacitor to the line.
Overhead lines use power capacitors which are generally in the form of batteries made up of unit capacitors coupled together in series and in parallel, and which are connected to the transport grid via a circuit breaker. A frame supports the batteries at a distance from the ground so that any risk of short circuit through atmospheric air between a battery and ground is avoided, particularly when the transport grid operates at a voltage of about 400,000 V. The impossibility of placing such batteries on the ground, and a fortiori of burying them, constitutes a problem for connecting a power capacitor to a gas-insulated line.